1. Technical Field
The invention disclosed broadly relates to the switching of multi-phase electrical power sources, and more particularly, relates to the precise matching of the relative phases in a multi-phase system for switching between power sources.
2. Background Art
Delta power systems and some unreferenced Y power systems are commonly used in power generation and distribution networks. These are normally three-phase power sources. The voltage waveforms of a three-phrase power system are shown in FIG. 1. Conventionally, three-phase voltage waveforms are represented by Phase A, Phase B and Phase C, which are generated so as to be 120 degrees in phase separation, respectively. The measurement of the relative phase relation between two sources in a multi-phase system is important in power switching control applications. For references to the U.S. Pat. No. 4,761,563 to Ross and Woodworth, entitled "Asynchronous Multi-Phase Switching Gear," wherein FIG. 1 shows a schematic of a functional block diagram of the system wherein a closely matched phase rotation detection unit and a coincidence detector are used when switching from one power source to an asynchronous second source. The devices most closely match the phase of the load to the second power source. The invention provides the input to the control unit as to the best load matching. By closely matching each phase of the load to the phase of the second source which is nearest in phase alignment, a near synchronous transfer of three-phase power from the first power source to the second power source can be made with minimum disruption to the load.
To implement an asynchronous multi-phase switching method it is necessary to resolve the relative phases of two asynchronous three-phase power sources such that an optimal phase to phase matchup may be made between them. Matchup in this case is defined as finding, for each phase (commonly labeled A, B or C) of the load, that phase of the second source which is closest in time (phase angle). There are only three possibilities as shown in FIG. 2 for matchups which fit the criterion of preserving the phase rotation direction. Phase rotation direction for this disclosure will always be taken to be to A to B to C, The problem demonstrates that given typical power sources with three phases at 120 degrees to each other, this "best of three" matchup yields a theoretical worst case angular discontinuity of 60 degrees. This is shown in FIG. 3. Note that the matchup must occur between the load and the "new" source. It is possible to monitor only the present and new source voltage polarities, however the solution must then include information about what the connection configuration is (1 of 3) between the present source and load. This configuration is called the "present path."
The problem, then, is to establish real time knowledge of the angle of rotation of the required waveforms of sufficient accuracy to calculate the best of three matchups with a tolerable worst case error. Error is due to the difference in source frequencies and source waveform sampling rate. This can be achieved with a logic system that receives as data the zero-voltage crossing of the three waveforms from each source or from the load and the "new" source.